In certain factory power distribution systems, relatively high-voltage power (i.e. greater than 1,000 volts) provided by the power company generation station may be stepped down to lower voltage power using a transformer. The lower voltage power may then be distributed around the factory to various power equipment such as, motors, welding machinery and large computers. Such power distribution systems of this type may be divided into branches, where each branch may supply power to a portion of the factory. The power distribution system is protected by installing low voltage fuses or circuit breakers in each branch so that a fault, such as a short circuit in a piece of equipment, supplied by one branch should not affect the power distributed to equipment coupled to the other branches. In addition to detecting large overcurrent conditions relating to short circuit faults, industrial circuit breakers may also detect long-time overcurrent conditions and excessive ground current. Relatively simple circuit breakers may be thermally tripped as a result of heating caused by an overcurrent condition, and is considered to be better for detecting relatively low level overcurrent conditions since it measures the cumulative heating effect of the low-level overcurrent condition over some time period. Such breakers may, however, respond too slowly to provide effective protection against high-current short circuit conditions.
Another type of circuit breaker monitors the current level being passed through the branch circuit and trips the breaker when the current exceeds a predefined maximum value. Such circuit breakers may include a microcontroller coupled to one or more current sensors. The microcontroller continually monitors the digitized current values using a curve which defines permissible time frames in which both low-level and high-level overcurrent conditions may exist. If an overcurrent condition is maintained for longer than its permissible time frame, the breaker is tripped. Although this breaker type is believed to provide protection against both long-time and short time overcurrent conditions, if it does not calculate Root-Mean Square (RMS) current values, it may erroneously trip the circuit when a nonlinear load, such as a welding machine, is coupled to the branch that it is protecting. Nonlinear loads may produce harmonics in the current waveform. These harmonics may distort the current waveform, causing it to exhibit peak values which are augmented at the harmonic frequencies. When the microcontroller, which assumes a sinusoidal current waveform, detects these peaks, it may trip the circuit breaker even though the heating effect of the distorted waveform may not require that the circuit be broken or otherwise interrupted.
Since the above described circuit breakers monitor overcurrent conditions, other types of faults such as over or under voltage conditions and phase imbalances may be missed unless or until they result in an overcurrent fault. Circuit protection for such faults may require special purpose line monitoring and relaying equipment, separate from the overcurrent breakers.
Another issue with certain existing circuit breakers involves the time required to restore the branch to operation once the breaker has tripped. For transient faults, such as a power surge during an electrical storm, a technician must go onto the factory floor, locate the tripped breakers and reset them. Depending on the technician's experience and knowledge, this may take a few minutes or a few hours. In this instance, however, the delay may be reduced by using a circuit breaker with an automatic recloser.
Faults caused by the equipment that is powered by the branch may be more difficult to locate. Certain circuit breakers may provide little if any information on the type of fault that caused the breaker to trip. Thus, the technician may need to install power monitors on each piece of equipment to determine if the fault was a long-time low-level overcurrent condition caused, for example, by a defective motor winding, or an intermittent short circuit fault. Such faults may take several days to locate and correct.
Another issue with existing low-voltage circuit breaker systems concerns the lack of effective backup protection if the circuit breaker fails to trip. This is more of a concern with microcontroller based trip units than with the older thermal trip units. In general, effective backup protection may include a fuse, in series with the branch line, which blows at a short-circuit current slightly higher than the short-circuit current of the breaker. If the microcontroller or any of its associated circuitry fails, a lower-level overcurrent condition may damage the distribution system and/or the equipment being protected before the backup fuse is blown.
Increasingly, the consumption of electrical power by a load is also monitored. Such power monitoring has been known at least since about the mid 1980s. As such, equipment manufacturers are increasingly using electronic circuit protection devices with circuit breaker units. These electronic circuit protection units may sample signals to provide various information, such as current, voltage, power factor, harmonics, kilowatt hours, var-hours, va-hours, instantaneous power, phase balance/imbalance, phase loading in relation to historical numbers and a percentage of maximum level. Moreover, these values may be stored to form a database.
Such information was only available in alpha-numeric displays at the power meter or electronic trip unit. An example of a graphical display interface for displaying power information of an electronic circuit device is U.S. Pat. No. 5,675,754 issued to King et al. and assigned to Siemens Energy & Automation, Inc., which is the assignee of the present application. Other forms of display were accomplished by down loading the relevant data to another computer either directly or in a network configuration.